1. Field of the Invention
This invention relates to a fixation sight apparatus for an ophthalmological instrument to be used for such ophthalmological instruments such as a retinal camera, a refractometer, a tonometer, a perimeter, etc. in order to have an eye to be tested fixedly sight or gaze at a fixation mark.
2. Description of the Related Art
Heretofore, there has been put into actual use an ophthalmological instrument for having a person to be tested fixedly sight or gaze at a fixation mark. In most ophthalmological instruments, a diopter correction mechanism is provided so that even if the eye to be tested is ametropic, myopic, or hyperapic the eye to be tested can fixedly sight the fixation mark.
Two different systems of diopter correction mechanisms are known; one is of the system wherein a plurality of correction lenses having different refractive powers with respect to one another are selectively inserted into an optical path of a fixation sight apparatus, and the other is of the system wherein either one or both of the fixation mark image and the projection lens adapted to project the fixation mark image to the eye to be tested are individually or are simultaneously moved in the optical axis direction thereof to optically position the fixation mark in a far point position of the eye to be tested. Particularly, the latter system is constituted as such that the eye can fixedly sight or gaze at the fixation mark with least accommodation of the eye by optically moving the fixation mark from a position nearer than the far point of the eye to a position farther than the far point position on the optical axis by moving the projection lens.
However, the conventional fixation sight apparatus for ophthalmological instruments including any of the above-mentioned diopter correcting mechanism is complicated in its structure due to the complicated structure of the diopter mechanism employed therein and also the employment of actuation means for actuating the projection lens, correction lens, etc.
Further, although the action of fixedly sighting the eye to be tested on the fixation mark to correct the diopter of the eye is secondary compared to the main measuring and testing actions of the ophthalmological instrument having the fixation sight apparatus, in spite of the foregoing, such a diopter correction operation to actuate the projection lens and the correction lens using the diopter correction mechanism is required to be effected every time the measurement and testing are carried out, which causes the measurer inconveniences.
Moreover, the results corrected by means of the diopter correction operation are required to be confirmed through question and answer between the measurer and the person to be tested. This confirmation action is troublesome. In addition, since the corrected results rely only on the response of the person to be tested, the accuracy thereof is dubious.